Need a little help with the math here as to how the Healing Amp. will stack. If I make a 10%, 20%, 30% Healing amp item, I know it doesn't stack additively- as far as I've heard, it's multiplicative stacking.

I'm trying to figure out if this is worthwhile to make the most out of the aura on my Clonk when she's raid healing- If my aura hits for 18 and I have a 10/20/30% Amp item, what would my aura hit for?

Thanks for help with the math- I'm not a numbers person.